With the proliferation of variable frequency drives (VFDs), modern induction motors are usually designed to run at various frequencies. However, voltage compensation is required as frequency changes. In the simplest mode, voltage changes in exact proportion to the frequency. In some industrial applications, we have run motors, with nominal nameplate ratings of 60Hz, all the way up to 90Hz with no problems. Operation at lower frequencies is generally a bigger problem. Firstly, the voltage must be reduced in proportion to the frequency. Secondly, motors with a fan operated off of the main motor shaft will suffer from much slower air circulation, and this is often the limiting factor. For very low speed operation, a different means of cooling may be required.